The present invention generally relates to fabrication of semiconductor devices and more particularly to a process for exposing a semiconductor pattern on an object such as a semiconductor substrate with an improved accuracy.
Electron beam lithography is a key process for fabricating advanced semiconductor integrated circuits having a very large integration density. With the use of electron beam lithography, a device pattern having a line width of less than 0.05 .mu.m can be fabricated easily with an alignment error of less than 0.02 .mu.m. Thus, the electron beam lithography is expected to play a major role in the fabrication of future integrated circuits such as DRAMs having a storage capacity of 256 Mbits or more.
In the fabrication of memory devices, the throughput of production is an essential factor, in addition to the resolution of the device patterning. In this respect, the electron beam lithography that uses a single, focused electron beam for the exposure, is disadvantageous as compared with the conventional optical exposure processes that expose the entire device pattern in one single shot. On the other hand, such a conventional optical exposure process is reaching its limit of resolution, and there is a situation that one has to rely upon the electron beam exposure process for the fabrication of the large capacity memory devices of the future.
Under such circumstances, various efforts are being made for improving the throughput of the electron beam exposure process. For example, the inventor of the present invention has previously proposed a so-called block exposure process wherein the device pattern is decomposed into a number of fundamental patterns and the electron beam is shaped in accordance with one of these fundamental patterns. With the use of the block exposure process, one can now achieve a throughput of about 1 cm.sup.2 /sec.
On the other hand, in the fabrication of semiconductor devices such as microprocessors that include complex, irregular patterns, there arises a problem in the block exposure system in that the efficiency of exposure is decreased significantly. In the block exposure processes, one can shape the electron beam to have a pattern that is selected from a combination of small, fundamental patterns, with high efficiency. When exposing an irregular pattern not included in the set of fundamental patterns, on the other hand, one has to shape the electron beam according to the variable-shaping process by energizing the electrostatic or electromagnetic deflectors with a controlled magnitude. However, such a variable-shaping process decreases the exposure throughput due to the setting time of the deflectors or operational speed of the control system that requires exact control of the deflectors.
In order to achieve a high exposure throughput in the fabrication of semiconductor devices that include complex, irregular patterns, the use of a so-called blanking aperture array is proposed. In the blanking aperture array, the exposure patterns are represented in the form of a dot pattern that includes small exposure dots. Thereby, the electron beam exposure system forms each of the dots by using a blanking aperture array that turns on and turns off the electron beam in correspondence to each of the exposure dots.
FIG. 1 shows the construction of a conventional electron beam exposure system that uses the technique of blanking aperture array. Referring to the drawing, the electron beam exposure system is generally formed from an electron optical system 100 for producing and focusing an electron beam and a control system 200 for controlling the optical system 100.
The electron optical system 100 includes electron gun 104 as a source of the electron beam. The electron gun 104 includes a cathode electrode 101, a grid electrode 102 and an anode electrode 103, and produces the electron beam generally in the direction of a predetermined optical axis O in the form of a spreading beam.
The electron beam thus produced by the electron gun 104 is passed through a shaping aperture 105a formed in an aperture plate 105. The aperture plate 105 is provided such that the aperture 105a is in alignment with the optical axis O and shapes the incident electron beam to have a rectangular cross section.
The electron beam thus shaped is received by an electron lens 107 that focuses the incident electron beam upon a blanking aperture array (designated hereinafter as BAA) mask 110. It should be noted that the lens 107 projects the image of the rectangular aperture 105a on the BAA mask 110. As will be described later with reference to FIG. 2, the BAA mask 110 carries a number of apertures in correspondence to a number of exposure dots formed on a semiconductor substrate, and each aperture is provided with an electrostatic deflector that is controlled in response to a control signal E. Thereby, the apertures on the BAA mask 110 allows the passage of the electron beam as it is when in the un-energized state. When energized, on the other hand, the electron beam is deflected from the optical axis O upon passage through the aperture. As a result, the substrate is formed with exposure dots in correspondence to the apertures that are in the unenergized state.
The electron beam thus passed through the BAA mask 110 is then focused at a point f1 that is located on the optical axis O after passing through electron lenses 108 and 116. There, the image of the addressed apertures on the BAA 110 is demagnified at the point f1. The electron beam thus focused is then passed through a round aperture 117a formed in a blanking plate 117 and further focused on the surface of a substrate 123 that is held on a movable stage 126, after passing through electron lenses 119 and 120 that form another demagnifying optical system. There, the electron lens 120 serves for an objective lens and includes various coils such as correction coils 120 and 121 for focusing compensation and astigmatic compensation as well as deflection coils 124 and 125 for moving the focused electron beam over the surface of the substrate 123.
In order to control the exposure operation, the electron beam exposure system of FIG. 1 includes the control system 200, wherein the control system 200 includes memory devices such as a magnetic tape device 201 and magnetic disk devices 202, 203 that are provided to store various data of the device pattern of the semiconductor device to be written. In the illustrated example, the magnetic tape device 201 is used for storing various design parameters, the magnetic disk device 202 is used for storing the exposure pattern data, and the magnetic disk device 203 is used for storing the pattern of the apertures on the BAA mask 110.
The data stored in the memory devices is read out by a CPU 204 and is transferred to an interface device 205 after data decompression. There, the data for specifying the pattern on the block mask 110 is extracted and stored in a data memory 206. The data stored in the data memory 206 is then transferred to a first control unit 207 that produces the foregoing control signal E and supplies the same to the deflectors provided on each aperture on the BAA mask 110. In response thereto, the energization of the apertures on the BAA mask 110 is controlled and hence the formation of the exposure dots on the substrate 123.
The first control unit 207 further supplies a control signal to a blanking control unit 210 that in turn produces a blanking signal for shutting off the electron beam. This blanking signal is then converted to an analog signal SB in a D/A converter 211 and the analog signal SB is supplied to a deflector 115 that causes a deflection of the electron beam away from the optical axis O. In response to this, the electron beam misses the round aperture 117a and disappears from surface of the substrate 123.
The interface device 205 further extracts and supplies the data for controlling the movement of the electron beam on the surface of the substrate 123 to a second control unit 212. In response thereto, the control unit 212 produces a control signal for controlling the deflection of the electron beam on the surface of the substrate 123 and supplies the same to a wafer deflection control unit 215 that in turn produces and supplies deflection control signals to D/A converters 216 and 217. The D/A converters 216 and 217 in turn, produce drive signals SW1 and SW2 for driving the deflectors respectively and supply the same to the deflectors 124 and 125 for causing the deflection of the electron beam. Thereby, the position of the stage 126 is detected by a laser interferometer 214 and the wafer deflection control unit 215 modifies the output deflection control signals and hence the drive signals SW1 and SW2 according to the result of measurement of the stage position by the laser interferometer. Further, the second control unit 212 produces a control signal that causes a lateral movement of the stage 126.
FIG. 2 shows an example of the BAA mask 110 that is used in the electron beam exposure system of FIG. 22.
Referring to FIG. 2, the mask 110 has a size b of about 1200 .mu.m in the scanning direction of the electron beam and a size c of about 3200 .mu.m in the direction perpendicular to the scanning direction. The mask 110 carries thereon a number of aperture rows 1A.sub.1 -1A.sub.n, 1B.sub.1 -1B.sub.n, 2A.sub.1 -2A.sub.n, . . . , 8A.sub.1 -8A.sub.n, 8B.sub.1 -8B.sub.n. In FIG. 2, the aperture row that includes the apertures 1A.sub.1 -1A.sub.n is designated as 1A, the aperture row that includes the apertures 1B.sub.1 -1B.sub.n is designated as 1B . . . Thereby, the apertures 1A.sub.1 -1A.sub.n form an A-group aperture row, while the apertures 1B.sub.1 -1B.sub.n form a B-group aperture row. In the mask 110 of FIG. 2, it should be noted that the apertures forming the B-group aperture rows such as 1B are formed with a displacement with respect to the apertures forming the A-group aperture rows, such as 1A, by one pitch. In the mask of the illustrated example, each aperture row includes 64 apertures each having a size S of 25 .mu.m for each edge and disposed with a pitch 2S of 50 .mu.m. Thereby, the BAA mask 110 carries thereon sixteen aperture rows 1A-8B such that the aperture rows are repeated in the scanning direction with a pitch of repetition of 2S. In correspondence to each aperture, a square dot pattern having a size of 0.08 .mu.m.times.0.08 .mu.m is formed on the substrate 123. Of course, the number of the apertures repeated in the scanning direction is not limited to sixteen but can be much larger such as one thousand or more.
In the conventional electron beam exposure system of FIG. 1 that uses the BAA mask 110, therefore, it will be noted that a number of flat electron beam arrays, each being an assembly of a number of electron beams aligned in the direction generally perpendicular to the scanning direction, are formed. By deflecting such flat electron beam arrays in the scanning direction, a semiconductor pattern is exposed on the semiconductor substrate 123 as an assembly of exposure dots. Thereby, by disposing the apertures alternately on the mask 110, one can avoid the electron beams from coming excessively close to each other, and the problem of Coulomb interaction between the electron beams can be avoided.
FIG. 3 shows the apertures on the BAA mask 110 and the electrostatic deflectors that cooperate therewith.
Referring to FIG. 3, each electrostatic deflector includes a ground electrode GND indicated by cross-hatching and a drive electrode ACT that opposes the ground electrode GND with the aperture intervening therebetween, and the BAA control circuit of FIG. 22 includes a plurality of drive circuits in correspondence to each of the aperture columns COL.sub.1, COL.sub.2, COL.sub.3, . . . , COL.sub.128. It should be noted that each aperture column includes a plurality of apertures aligned in the scanning direction in FIG. 3. As will be described in detail later, those apertures aligned to form an aperture column such as the column COL.sub.2 are supplied with the same drive signal consecutively with a delay that corresponds to the beam scanning speed. Thereby, a square dot pattern having a size of 0.08 .mu.m .times.0.08 .mu.m is exposed repeatedly on the substrate 123 by the electron beam that has passed through the apertures. In order to achieve such a multiple exposure, the apertures are supplied, in each aperture column, with the drive signal via respective lines. FIG. 3 shows such a line collectively by a symbol L. Illustrations of such, lines corresponding to the other aperture columns are omitted for the sake of simplicity.
FIG. 4 shows an example of the exposure pattern formed on the substrate 123 by means of the electron beam shaped by the BAA mask 110 of FIG. 2. Referring to FIG. 4, it will be noted that the exposure pattern is formed of a two-dimensional array of the exposure dots of A-group corresponding to the A-group apertures such as 1A.sub.1 and the exposure dots of B-group corresponding to the B-group apertures such as 1B.sub.1. Each exposure dot typically has the size of 0.08 .mu.m.times.0.08 .mu.m as described before. Thereby, by turning on and turning off the exposure dots by controlling the BAA mask 110, it becomes possible to modify the exposure pattern with the smallest area of 0.08.times.0.08 .mu.m.
In the electron beam exposure process described heretofore, it should be noted that the exposure pattern is written on the substrate 123 by causing a photochemical reaction in the photoresist that covers the surface of the substrate 123 as a result of radiation of the electron beam. Thereby, the incident electron beam induces a backscattering of electrons at the substrate 123, and these backscattered electrons cause a photochemical reaction in the photoresist.
FIGS. 5(A) and 5(B) show an example of exposure of various patterns on the substrate 123 achieved by the apparatus of FIG. 1.
Referring to FIG. 5(A), the drawing shows the energy or dose of the electron beam supplied to an electron beam resist that covers the surface of the substrate for a case where the exposed device pattern designated as P.sub.1 has a pattern density a close to 100%. The pattern density herein means the percentage of the region of the substrate that is exposed by the electron beam. In FIG. 5(A), the threshold of exposure energy is represented as TH. When the dose of the electron beam exceeds the foregoing threshold TH, the exposure of the electron beam resist occurs. On the other hand, when the dose does not reach the threshold TH, no exposure is made.
In the exposure of FIG. 5(A), it should be noted that there is a substantial background exposure as represented by .beta., which is caused by the backscattering of electrons from the substrate. Such a background exposure is known as the "proximity effect." When such a background exposure occurs, the dose of the electron beam for the device pattern P.sub.1 increases inevitably as indicated in FIG. 5(A). As a natural consequence, such a background exposure does not occur in small, isolated patterns such as a pattern P.sub.2 where the backscattering of the electrons is small. Thereby, there occurs a difference in the dose of exposure between the region where a dense pattern is exposed and the region where a small isolated pattern is formed.
A similar change of dose occurs also between the device patterns having a large pattern density such as the pattern P.sub.1 and the device patterns having less dense pattern density such as a pattern P.sub.3 shown in FIG. 5(B). The device pattern P.sub.3 of FIG. 5(B) has a pattern density of 50%, for example, and there occurs a backscattering of the electrons with a magnitude represented as .alpha..beta..
Under such a situation, it will be understood that the variation of exposure, caused by the backscattering of electrons, has to be compensated for such that there is a uniform background exposure. Otherwise, there occurs a case where small isolated patterns such as the pattern P.sub.2 of FIG. 3(B) are not exposed.
In order to compensate for the proximity effect, there is a proposal to expose the pattern on the substrate a plurality of times. However, such a multiple exposure process generally includes the step of scanning the substrate in more than three or four times, and because of this, the conventional process for compensating for the proximity effect has caused a problem of low exposure throughput. It should be noted that the electron beam exposure process is conducted on the substrate in the form of parallel stripes such that the exposure starts at an end of a stripe and proceeds to the other end of the stripe by scanning the electron beam along the stripe. When the exposure for that stripe is completed, the exposure for the next stripe is repeated. In such an exposure process, one cannot increase the speed of scanning beyond a certain limit because of the limitation in the response speed of the laser interferometer that drives the stage of the substrate. Further, an excessive increase in the scanning speed tends to cause a vortex current to flow through the substrate as a result of leak flux of the electron lens interlinking with the substrate. Thereby, such a vortex current tends to cause a deviation in the electron beam on the substrate. In addition, the repetition of the exposure process, more than three or four times, increases the amount of information to be processed for preparing the exposure data that is used in each exposure process. In view of the limitation in the data transfer rate of the control system 200, however, it is difficult to repeat the exposure process more than twice.